diff --git a/requests/models.py b/requests/models.py
index 87bb8241..eeab8e27 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -94,8 +94,10 @@ class RequestEncodingMixin(object):
         if parameters are supplied as a dict.
 
         """
-        if (not files) or isinstance(data, str):
-            return None
+        if (not files):
+            raise ValueError("Files must be provided.")
+        elif isinstance(data, basestring):
+            raise ValueError("Data must not be a string.")
 
         new_fields = []
         fields = to_key_val_list(data or {})
diff --git a/test_requests.py b/test_requests.py
index e62d9237..714c32b6 100755
--- a/test_requests.py
+++ b/test_requests.py
@@ -10,6 +10,7 @@ import unittest
 import pickle
 
 import requests
+import pytest
 from requests.auth import HTTPDigestAuth
 from requests.adapters import HTTPAdapter
 from requests.compat import str, cookielib, getproxies, urljoin, urlparse
@@ -339,6 +340,12 @@ class RequestsTestCase(unittest.TestCase):
         except ValueError:
             pass
 
+    def test_conflicting_post_params(self):
+        url = httpbin('post')
+        with open('requirements.txt') as f:
+            pytest.raises(ValueError, "requests.post(url, data='[{\"some\": \"data\"}]', files={'some': f})")
+            pytest.raises(ValueError, "requests.post(url, data=u'[{\"some\": \"data\"}]', files={'some': f})")
+
     def test_request_ok_set(self):
         r = requests.get(httpbin('status', '404'))
         self.assertEqual(r.ok, False)